In radio communications networks, traditionally, a given geographical area is divided into cells. Each cell of a network (thus each mobile communication device within a cell) is served by a base station. The operation of the base stations is controlled by a base station controller, which might be implemented as part of the base stations or as a separate entity. The base stations are capable of dynamically adjusting their transmission power and/or direction to ensure optimum radio channel quality to the served mobile communication devices (such as mobile telephones and other user equipment).
In relatively small geographical areas, such as office buildings or similar, where additional network capacity or a specific set of services are needed, so-called ‘pico’ cells may be implemented within (or partially overlapping with) the regular (i.e. ‘macro’) cells of the radio network. Pico cells are also known as ‘femto’ cells or simply ‘small’ cells. In some cases, a pico cell covers a home or a single room only. Pico cells effectively form parts of a larger network infrastructure, while providing services within a smaller coverage area. Communication networks comprising a variety of base station types and cell sizes (e.g. both macro and pico cells) are often referred to as Heterogeneous Networks (HetNets).
In order to maximise usage of system resources in HetNets, transmissions by base stations serving such macro and pico cells need to be synchronised in order to avoid or reduce harmful interference between them and interference caused to mobile communication devices served by the base stations. Therefore, the power of transmission may be selected by the base stations so that a maximum number of mobile communication devices can be served in parallel and at an optimum data rate without compromising transmission quality. Transmission power can be controlled on a base station level or cell level, whilst specific power levels can also be assigned to each mobile communication device and/or communication units (such as frames, subframes, resource elements, resource blocks, symbols) used to exchange data between the base stations and mobile communication devices.
In order to assist their serving base station in adjusting its transmission power appropriately, the mobile telephone is configured to measure and report the quality of the signals transmitted in its serving cell (and/or in neighbouring cells) and also to measure and report back any interference experienced in the serving cell. In order for the mobile telephone to able to measure interference caused by other transmitters than the serving base station, the serving base station's transmissions need to be muted, temporarily, at least for the duration of the mobile telephone's measurements.
The so-called Almost Blank Subframe (ABS) concept has been introduced by 3GPP as part of its Enhanced Inter-Cell Interference Coordination (eICIC) solution. ABS subframes are transmitted in accordance with a predetermined ABS pattern. Effectively, ABS subframes are special subframes designed to alleviate inter-cell interference, such as interference caused by a macro base station to a pico base station (and vice versa). The main feature of this technique is that in subframes designated to be an ABS subframe, the macro base station does not transmit any signals other than reference signals (at a very low power level). Therefore, if ABS subframes are configured in a macro cell, then the mobile communication device served by the pico cell (falling within or overlapping with the coverage area of that macro cell) is able to send data during such ABS subframes and hence avoid interference from the macro cell (at least for the duration of the ABS subframes).
More recently, 3GPP has also introduced a technique referred to as the Further Enhanced Inter-Cell Interference Coordination (feICIC). Briefly, instead of transmitting only reference signals in the ABS subframes, feICIC allows limited data transmissions as well. Specifically, feICIC allows transmissions on the physical downlink shared channel (PDSCH), albeit at a reduced power level (which may still be adequate for e.g. mobile communication devices located relatively close to the base station). Thus feICIC may reduce waste of available cell capacity (compared to eICIC), since transmissions in the macro cell are still possible during feICIC compatible ABS subframes (although not for all mobile communication devices).
In more detail, feICIC in 3GPP Release 10 and 11 requires the macro cell base station to configure its (semi-static) ABS pattern, and to inform its neighbouring pico base station(s) (using the so-called X2 interface) about its ABS configuration. The ABS configuration includes information identifying the applicable ABS muting pattern in the macro cell. The ABS muting pattern is expressed using a 40-bit string (representing a 40 ms periodicity) for frequency-division duplexing (FDD) mode.
The small cell (pico base station) uses the ABS information from the macro base station at least:                in its scheduling decisions (especially for those mobile communication devices that are located in the so-called cell range expansion (RE) area); and/or        in configuring measurement restrictions for mobile communication devices served by the pico cell (for example, through appropriate Channel State Information (CSI) measurement restrictions).        
Pico cells are typically provided using passive antennas, which cause the overlaid coverage area of the macro cell over the small cell to remain relatively constant from control and data channel perspectives. This in turn makes it necessary in HetNet co-channel (macro/pico) deployments to use a strict ABS blinding (muting) of transmissions in the cells of the macro base station in the time-domain (i.e. the respective ABS patterns must be shared and co-ordinated between the macro and the pico base station). In other words, it is necessary for the overlapping macro and small cells to respect the semi-statically configured ABS pattern and transmit only such signals that are allowed in the ABS subframes.